If you need counts for several letters it would be good to use array of counters:
1) initialize array A to zeroes
2) read next letter C
3) convert C to integer value X (not pretty tough in C, ha-ha)
4) increment A[X] by one
5) if there are more letters, go to 2
6) at the end run through A and for each A[I] convert I back to character C and print out how many times this C was counted (that is A[I])